Yes it is diluted, but you inject 5 units rather than 2.5, so you end up with the same amount. That volume of water is used because there is no 2.5 line on the syringe, but there is a 5 line. Make sense? Reta is said to last about 6-8 weeks. Use proper BAC water, reconstitute under the cleanest conditions you can, do not shake it, keep it away from temperature swings (the fridge door has lots) and light, and it should reach about 8 weeks. Once expired it only weakens, so you stop feeling effects. If it turns cloudy or changes color, that is BAD, but only a compromised vial would do that. Toss it if it stops working. In your example, with 0.5mg at the start, you might waste some if you never go higher, yet that waste happens anyway when there are no effects.